raise_error supports composable matchers:

• The first arg is matched against the exception object, supporting things like this.
• The 2nd arg is matched against the exception message, supporting things like a_string_including(...).

Sometimes you only care about matching on the string, though. raise_error partially supports this by allowing you to pass a regex as the first argument. I think we should similarly support a matcher matching against the message as the first argument. Basically, we would change raise_error so that when it receives a single argument, it passes if that argument matches against either the exception object itself or the message.